Usually, casings for food are classified into natural and artificial casings. Natural casings are produced from the intestines of domestic animals such as sheep, swine, cattle and the like. On the other hand, artificial casings include edible casings (collagen proteins) and non-edible casings (cellulose, plastics).
Since the present invention is directed to edible casings, conventional processes for producing them are described below.
Natural casings are produced by removing the intestines from an animal, removing feculence and mucosa therefrom by washing with water and salting.
As for edible casings (collagen proteins), it was reported that Karl Freudenberg Co. in Germany first succeeded in the production of a thick artificial casing material by using collagen fibers of the cattle skin in the middle of 1920's and Becker Co. marketed this material under the trade name of "Naturin Casing". Such casings have been produced and sold by several companies in different countries. Among them, a typical process is described below.
According to this process, the dermis of an adult cow is soaked in lime, followed by deliming, pulverizing mechanically and disentangling after being swollen well with an acid to prepare a collagen fiber paste. On the other hand, the purified cattle dermis is subjected to enzymatic decomposition with a proteolytic enzyme, "Procuase" (Meiji Seika Co., Ltd.), to prepare an enzymatically digested collagen. Eighty parts of the collagen fiber paste previously prepared is mixed with 20 parts of the enzymatically digested collagen, and the mixture is extruded into a saturated saline solution to form a molded material by coagulation. After coagulation, the molded material is tanned, washed with water, soaked in glycerin bath, and dried. After drying, the material is folded in bellows, heated for several hours (below 100.degree. C.) and allowed to stand in a constant humidity to obtain a casing.
As described above, in the production of a conventional tubular edible casing, a coagulant has been always used and no proteinaceous film which is an edible tubular casing produced from soybean protein has been produced heretofore in the prior art.
Conventional natural casings and edible casings have the following problems:
As for natural casings, it is difficult to remove feculence and mucosa from the animal intestines by washing. Moreover, there have been problems in quality such as pinholes and different size.
As for edible casings, the preparation of a paste from collagen fibers in the pretreatment for extrusion from a rotary nozzle into a saturated saline solution is complicated and requires much labor and time with low productivity. In addition, the raw materials are expensive.